1. Field of the Invention
The present invention relates to an apparatus for producing a single crystal by pulling a single crystal from source melt by means of the Czochralski method (hereinafter referred to as the "CZ method"), and especially, relates to an apparatus for producing a single crystal by means of the CZ method, suitable for pulling a heavy single crystal having a diameter of greater than 300 mm.
2. Description of the Related Art
A single crystal silicon is generally produced through use of the CZ method. According to the CZ method, polycrystalline silicon is charged into a quartz crucible disposed within a single crystal pulling apparatus. The polycrystalline silicon is heated and dissolved into a melt by means of a heater disposed around the quartz crucible. Subsequently, a seed crystal attached to a seed chuck is immersed into the melt. The seed holder is pulled while the seed chuck and the quartz crucible are rotated in a single direction or in opposite directions, to thereby grow a single crystal to a predetermined diameter and length. Under the CZ method, a neck having a diameter of about 3-4 mm must be formed in a lower portion of the seed crystal by means of the Dash technique in order toeliminate dislocations from the seed crystal. However, in response to a recent tendency for improving the efficiency of production of a semiconductor device, a demand exists for manufacture of a single crystal having a large diameter by means of the CZ method. In association with an increase in the diameter of a single crystal, the weight of the single crystal increases. Single crystal manufacturing apparatus have been suggested (in, for example, Japanese Patent Application Laid-Open Nos. Sho-62-288191 and 63-252991) , in which a constricted engagement step is formed in a single crystal remaining in a pulled state so that the heavy single crystal can withstand pulling action.
FIG. 18 shows an example of a conventional single crystal manufacturing apparatus provided with a clamping body. A pull head 2, which is rotatably provided at the upper end of the manufacturing apparatus, has provided therein a crystal pull wire take-up device 3 and a plurality of wire take-up devices 51 for raising/lowering a clamping member 50. A seed chuck 6 holding a seed crystal 5 is fixed to the lower end of a crystal pull wire 4 hanging the crystal pull wire take-up device 3 and extends to the center of a furnace. Melt 7 is stored in a quartz crucible which is disposed within an unillustrated main chamberso as to be able to rotate and move vertically.
The clamping member 50 is provided with a plurality of claws 53 provided in a pivotable manner at the lower end of a cylindrical clamping body 52 (the claws 53 are pivotable within a vertical plane and within a predetermined range of angle). The claws 53 are engaged with an engagement step engagement stepla. By means of this arrangement, a single crystal 1 can be pulled when the wire take-up devices 51 takes up wires 54.
However, the single crystal manufacturing apparatus having the foregoing configuration often encounters a problem of occurrence of an inclination in the single crystal 1 for reasons of an inclination in the gripping member 50 stemming from a variation in extension of the wires 54 (FIG. 19) or for reasons of an offset in points clamped by the gripping member 50 (FIG. 20). If the single crystal 1 is held in a slanted attitude and rotated while the axis of the single crystal 1 is tilted, runouts arising in a growth boundary is amplified, thereby adversely affecting the growth of the single crystal 1 and inducing poly-crystallization of the single crystal 1 arises.
Furthermore, in the event that the clamping member 50 clamps the single crystal 1 at the engagement step 1a with the aid of jaw-shaped claws 53, the weight of the single crystal 1 will focus on several contact points. This will induce cracking or distortion to the single crystal 1. On the worst occasion, the single crystal 1 could be destroyed.